This invention relates generally to print hammer actuators, and more particularly, to apparatus by which rebounding motion of a print hammer or actuator is efficiently and quickly damped without degrading the rest portion so as to carefully control flight time.
One of the limitations in achieving faster printing rates in high speed impact printers has been the settling time required by the print hammer or actuator before its next energization. Settling is required to accurately attain a position from which the actuator starts. If the actuator or hammer is not in its expected at-rest position, the flight time differs from the design dimensions and the time of hammer impact varies with respect to the moving type element. Misregistration of printing then occurs.
A backstop or bumper is provided against which the actuator can come to rest after being released from energization that is usually electromagnetic. In commercially available printing, the backstop is frequently an adjustable surface that is used to prevent wear and reduce noise while providing mediocre energy-absorbing qualities. A typical material used as a bumper is polyurethane. This will relatively slowly absorb and decrease the kinetic energy in the actuator compared to high damping materials such as butyl rubber.
The actuator rest position and flight time can tend to change with use. The backstop surface can wear and the energy-absorbing material can slowly cold flow or take a "set" in response to the repetitive pounding that it receives. The wear usually occurs because of relative movement between the backstop and actuator at the point of impact. This motion, even though a small amount, causes eventually change in the at-rest position of the actuator and results in longer flight times in the mechanisms. When the energy-absorbing material takes a set, its spring rate and damping chacteristics will be altered and further travel of the actuator will result. Also, changes in temperature cause expansion or contraction of the bumper material resulting in changes in rest position that affects flight time. In either instance adjustment or replacement is necessary and the large number of parallel hammer assemblies in each unit make corrective action costly and time consuming.